1. Field of the Invention
The present disclosure relates to liquid crystal display driver integrated circuits (LDI), and more particularly, to a charge pump circuit of an LDI.
2. Description of the Related Art
As shown in FIG. 1, a conventional liquid crystal display driver integrated circuit (LDI) for mobile equipment includes a source line driving voltage generating circuit 11 that generates a source line driving voltage SVDD, a common electrode driving voltage generating circuit 13 that generates a common electrode driving voltage VCOM, a gamma driving reference voltage generating circuit 15 that generates a gamma driving reference voltage (GVDD), and an LCD on/off voltage generating circuit 17 that generates an LCD on/off voltage VON/VOFF, in order to drive an LCD. Each of the source line driving voltage generating circuit 11, the common electrode driving voltage generating circuit 13, and the gamma driving reference voltage generating circuit 15 includes amplifiers.
Further, the LDI includes a charge pump circuit 19 to supply a power voltage to the circuits. The charge pump circuit 19 receives an external battery source voltage VCI of about 2.5 volts to 3.3 volts and then boosts the external battery source voltage VCI to produce a boost voltage AVDD of about 5.5 volts, and supplies the boost voltage (AVDD) to the circuits as source voltages.
Meanwhile, the LDI of FIG. 1 performs power-on operations in the sequence as illustrated in FIG. 2 to stably supply a source voltage to each of the circuits 11, 13, 15 and 17. After a reset, the charge pump circuit 19 is turned on to generate the boost voltage AVDD, and the LCD on/off voltage generating circuit 17 generates the LCD on voltage VON and the LCD off voltage VOFF in operations S1 and S2, respectively.
Next, the common electrode driving voltage generating circuit 13 and the gamma driving reference voltage generating circuit 15 are turned on by the boost voltage AVDD as a source voltage, and generate the common electrode driving voltage VCOM and the gamma driving reference voltage GVDD, respectively, in operations S3 and S4.
Finally, in a display-on mode in which an LCD panel is driven, the source line driving voltage generating circuit 11 is turned on in operations S5 through S7. The display-on mode may be a binary mode, which is a black and white mode, or a gradient mode, which is a color mode.
All of the amplifiers in the source line driving voltage generating circuit 11 are turned off in the binary mode, and are turned on in the gradient mode. Thus, the largest number of amplifiers operates in the gradient mode so that a load of an output node of the charge pump circuit 19 is maximized in the gradient mode. That is, the load derived from the boost voltage AVDD is greatest in the gradient mode.
In a conventional charge pump circuit used in an LDI, the current driving capability of a driver, such as a driving transfer transistor, which transfers a boost voltage generated in the charge pump circuit to an output node, is fixed based on the gradient mode of the display-on mode, in which the output node has a maximum load.
Accordingly, the driver consumes excess current even in an operational mode in which the charge pump circuit has a smaller load than in the gradient mode.